Integrated optical circulator enabling polarization diversity

ABSTRACT

A photonic integrated circulator can be fabricated by including a plurality of polarizing beam splitters and optical polarization rotators such that two copies of the optical signal are output at a receiver in substantially aligned polarization states. The circulator can be used for facilitating bi-directional communications between photonic integrated circuit devices, which are inherently polarization sensitive, while reducing signal loss.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of, and priority to, U.S.Provisional Application No. 62/595,539, titled “Integrated OpticalCirculator Enabling Polarization Diversity”, and filed on Dec. 6, 2017,the entire contents of which are hereby incorporated by reference forall purposes.

BACKGROUND

Today's optical communication networks require the management of a largenumber of optical fiber interconnections. In many of theseinterconnections, the transmitted and received signals are run in twoseparate fibers. Bi-directional communication, where the upstream anddownstream signals are running inside a single fiber, is an effectiveway to reduce the number of fibers used in a fiber-optic communicationnetwork. The key for implementing such bi-directional communications isto effectively separate and combine the upstream and downstream signals.One approach to separating and combining upstream and downstream signalsis to add optical circulators to each end of a fiber.

SUMMARY

At least one aspect is directed to an integrated optical circulatorenabling polarization diversity. The integrated optical circulatorincludes a first port configured to receive a first optical signal. Theintegrated optical circulator includes a second port configured totransmit the first optical signal received at the first port and alsoconfigured to receive a second optical signal. The integrated opticalcirculator includes a first polarizing beam splitter configured toreceive the second optical signal, split the second optical signal intoa first optical signal component and a second optical signal component,direct the first optical signal component towards a first reflectivesurface wherein the first reflective surface is configured to furtherdirect the first optical signal component to pass through a firstoptical polarization rotator, and direct the second optical signalcomponent through a second optical polarization rotator. The integratedoptical circulator includes a second polarizing beam splitter configuredto receive the second optical signal component after the second opticalsignal component passes through the second optical polarization rotator,and direct the second optical signal component towards a secondreflective surface. The second reflective surface directs the secondoptical signal component towards a third optical polarization rotator.The integrated optical circulator includes a third configured totransmit the first optical signal component of the second optical signalafter it passes through the first optical polarization rotator, thethird port transmitting the first component at a first polarizationstate. The integrated optical circulator includes a fourth portconfigured to receive the second component of the second optical signalafter it passes through the third optical polarization rotator, thefourth port transmitting the second optical signal component at a secondpolarization state substantially aligned with the first polarizationstate.

These and other aspects and implementations are discussed in detailbelow. The foregoing information and the following detailed descriptioninclude illustrative examples of various aspects and implementations,and provide an overview or framework for understanding the nature andcharacter of the claimed aspects and implementations. The drawingsprovide illustration and a further understanding of the various aspectsand implementations, and are incorporated in and constitute a part ofthis specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are not intended to be drawn to scale. Likereference numbers and designations in the various drawings indicate likeelements. For purposes of clarity, not every component may be labeled inevery drawing. In the drawings:

FIG. 1 shows a schematic of an example of a bi-directional communicationsystem implemented with circulators, according to an illustrativeimplementation;

FIG. 2 shows a schematic of an example of an integrated opticalcirculator, according to an illustrative implementation;

FIG. 3 shows a schematic of a further example of an integrated opticalcirculator, according to an illustrative implementation and

FIG. 4 shows a schematic of a further example of an integrated opticalcirculator configured for use with multiple transceivers, according toan illustrative implementation.

DETAILED DESCRIPTION

The various concepts introduced above and discussed in greater detailbelow may be implemented in any of numerous ways, as the describedconcepts are not limited to any particular manner of implementation.Examples of specific implementations and applications are providedprimarily for illustrative purposes.

Today's optical communication networks require the management of a largenumber of optical fiber interconnections. In many of theseinterconnections, the transmitted and received signals are run in twoseparate fibers. Employing bi-directional communication, where thetransmitted and received signals are running inside a single fiber, canreduce the number of fibers used in a fiber-optic communication network.The key for implementing such bi-directional communications is toeffectively separate and combine the upstream and downstream signals ateach end of the fibers.

One approach to establishing bi-directional communications is to addoptical circulators to each end of a fiber. Optical circulators aretypically three port devices which have a cyclic connectivity. Forinstance, an input optical signal enters port 1 of the circulator and isdirected to port 2, while another input optical signal enters port 2 andis directed to port 3. To achieve this functionality, the circulator isequipped with a combination of polarizers,non-reciprocal/magneto-optical materials such as Garnet, and phaseretarders.

FIG. 1 illustrates a bi-directional communication system 100 implementedwith optical circulators. The Bi-directional communication system 100includes a first transceiver 110, a second transceiver 120, a firstcirculator 130, a second circulator 140, and an optical fiber 150. Thefirst transceiver 110 includes a first transmitter 112 and a firstreceiver 114. Similarly, the second transceiver 120 includes a secondtransmitter 122 and a second receiver 124. The first circulator 130includes a first port 131, a second port 132 and a third port 133.Circulator 140 includes a fourth port 141, a fifth port 142 and a sixthport 143. The first port 131 is optically coupled to the firsttransmitter 112, while the third port 133 is optically coupled to thefirst receiver 114. Similarly, the fourth port 141 is optically coupledto the second transmitter 122, while the sixth port 143 is opticallycoupled to the second receiver 124. The second port 132 and the fifthport 142 are connected by an optical fiber 150.

The first transmitter 112 transmits an optical signal that enters thefirst port 131. The first circulator 130 directs the optical signalentering the first port 131 to exit the second port 132. The opticalsignal then propagates through the optical fiber 150 and enters thefifth port 142 of the second circulator 140. Once the optical signalenters the fifth port 142, the second circulator 140 directs the opticalsignal to exit the sixth port 143, where the second receiver 124receives the signal. Similarly, the second transmitter 122 transmits anoptical signal to the fourth port 141 of the second circulator 140. Thesecond circulator 140 directs the optical signal entering the fourthport 141 to exit the fifth port 142. The optical signal then propagatesthrough the optical fiber 150 and enters the second port 132 of thefirst circulator 130. Once the optical signal enters the second port132, the first circulator 130 directs the optical signal to exit thethird port 133, where the first receiver 114 receives the signal. Asshown in this example, attaching the circulators at each end of thefiber eliminates the need to use two fibers to connect the transmittersand receivers of the two transmitters. In addition to reducing thenumber of optical fibers needed for optical communication networks,optical circulators can improve the efficiency of networks employingoptical circuit switching technologies.

One disadvantage of adding traditional circulators to a fiber opticnetwork is that realization of their advantages requires addingcirculators external to the optical transceivers, which can increasecost, size, and insertion loss of the fiber optic link. However,integrated silicon photonics is a developing technology in the field ofoptical communications which promises low power and low cost photonicintegrated circuits by leveraging low cost, high yielding siliconcomplementary metal-oxide-semiconductor (CMOS) integrated circuitfoundries to fabricate integrated photonics in silicon, where thevarious transmitter and receiver devices of the transceiver would becombined in a single chip. These photonic integrated circuits form theengine of the optical transceiver. Thus, it may be preferable toconfigure a circulator so that it can be integrated with the photonicintegrated circuit in order to eliminate the need to add circulatorsexternal to the optical transceiver.

Integrated components have great potential to reduce the cost and sizeof fiber optic networks. However, many critical devices utilized insilicon photonics, such as waveguide based devices, are sensitive topolarization states. This means that the characteristics of the devicevary based on the polarization of the light being used within thedevice. For example, many types of Mach-Zehnder based Rx demultiplexer(demux) devices are polarization sensitive because the effective indexof the waveguide is dependent on the polarization. Consequently, theintegrated devices work more efficiently when receiving optical signalspropagating in one polarized state than when they receive opticalsignals in another polarized state. In the case of silicon photonics,devices work more efficiently when receiving optical signals propagatingin a p-polarized state as opposed to s-polarized signals. Thus, it isdifficult to design integrated devices to be polarization diverse inmost standard CMOS processes. Although silicon photonic IC transmitterscan be configured to transmit an optical signal in a specific polarizedstate, the optical signal typically does not reach a correspondingreceiver in the same polarized state. This is because the optical signalcan become depolarized as it propagates through a fiber due to theinherent properties of the fiber.

Traditional circulators utilize birefringent materials to separate theorthogonal polarizations of an incoming optical signal. However, as theoptical signal propagates through the circulator, the orthogonalpolarizations are recombined by further birefringent materials. Thus,the optical signal carries mixed polarizations as it exits thecirculator and is received by the transceiver. But, as discussed above,the integrated silicon devices may not work efficiently with thes-polarized component of the optical signal, and an unfavorable amountof optical signal loss will occur. Consequently, it is unfavorable tointegrate traditional circulators with silicon photonic ICs.Accordingly, it is advantageous to design a device that would enablebi-directional communications between photonics IC-based transceivers,while addressing the polarization sensitivity of the silicon baseddevices.

One way for achieving bi-directional communication with silicon photonicintegrated devices, while addressing the polarization sensitivity of thesilicon based devices, is to use a traditional external circulator withan integrated grating coupler in the silicon photonic IC. Certaingrating couplers have been shown to separate orthogonal polarizationmodes. However, many grating couplers have a high fundamental opticalloss. Also, as discussed earlier, the use of an external circulator canbe undesirable due to added size, cost and optical loss associated withthe circulator. Therefore, it may be preferable to construct anintegrated device that allows the combination of bi-directional linkswith silicon photonic technologies for lower cost, high density opticalinterconnects. This disclosure proposes an integrated circulatorconfigured to split the polarized states of an input signal into twocomponent signals, and transmit the component signals to separate portswith their polarization states substantially aligned.

Devices according to this disclosure can address the issues associatedwith enabling bi-directional communications between photonic IC-basedtransceivers primarily in several ways. For example, in terms of size,the subject device is designed small enough such that it can beintegrated with the photonic IC-based transceiver. Additionally, inorder to reduce optical loss, the subject device's polarizationsplitting/combining components are configured in such a manner as tosplit the mixed polarization optical signal input into two separateoptical signal components with orthogonal polarizations, and furtherrotating these separate optical signals to produce an output thataccommodates the signal polarization requirements of the siliconphotonics IC. To be sure, by separating the mixed polarization opticalsignal input into two signal components, and providing the signalcomponents to an integrated transceiver in substantially the samepolarization state, signal loss is reduced and both signal componentsare provided to the photonic chip in a polarized state that allow foreffective demultiplexing and other functions being implemented in thechip.

FIG. 2 shows a schematic of an example of an integrated opticalcirculator 200, according to an illustrative implementation. Theintegrated optical circulator 200 can be used to implementbi-directional, polarization diverse optical links using siliconphotonics. The integrated optical circulator 200 includes a first port210, a second port 211, a third port 212, and a fourth port 213. Theintegrated optical circulator also includes a first polarizing beamsplitter 220, a first reflective surface 221, a second polarizing beamsplitter 222 and a second reflective surface 223. The first polarizingbeam splitter 220 and the second polarizing beam splitter 222 may bepolarizing beam splitters known in the art, such as polarizing cube beamsplitters composed of a birefringent medium. In some embodiments, thefirst reflective surface 221 or the second reflective surface 223 arepolarizing beam splitters. In other embodiments, both are polarizationbeam splitters. The integrated optical circulator also includes a firstoptical polarization rotator 230, a second optical polarization rotator231, a third optical polarization rotator 232, a fourth opticalpolarization rotator 233 and a fifth optical polarization rotator 234.

The optical circulator 200 is a free space optical component. The secondport 211 is positioned to receive a second optical signal 241. In someimplementations, the second optical signal 241 includes a first opticalsignal component 241 a and a second optical signal component 241 b. Thefirst polarization beam splitter 220 is optically coupled with thesecond port 211 and positioned to receive the second optical signal 241after the second optical signal 241 propagates through the second port211. The third optical polarization rotator 232 is optically coupledwith the first polarizing beam splitter 220 and positioned to receivethe second optical signal component 241 b after it transmits through thefirst polarizing beam splitter 220. The fourth optical polarizationrotator 233 is optically coupled with the third optical polarizationrotator 232 and positioned to receive the second optical signalcomponent 241 b after it propagates through the third opticalpolarization rotator 232. The second polarizing beam splitter 222 isoptically coupled with the fourth optical polarization rotator 233 andpositioned to receive the second optical signal component 241 b after itpropagates through the fourth optical polarization rotator 233. Thesecond reflective surface 223 is optically coupled to the secondpolarizing beam splitter 222 and positioned to receive the secondoptical signal component 241 b after it is reflected by the secondpolarizing beam splitter 222. The fifth optical polarization rotator 234is optically coupled with the second reflective surface 223 andpositioned to receive the second optical signal component 241 b after itpropagates through the fifth optical polarization rotator 234. The firstport 240 is optically coupled with the second polarizing beam splitter222 and positioned to receive a first optical signal 240. The firstreflective surface 221 is optically coupled with the first polarizingbeam splitter 220 and positioned to receive the first optical signalcomponent 221 a after it is reflected by the first polarizing beamsplitter 220. The first optical polarization rotator 230 is opticallycoupled with the first reflective surface 221 and positioned to receivethe first optical signal component 241 a after it is reflected by thefirst reflective surface 221. The second optical polarization rotator231 is optically coupled with the first optical polarization rotator 230and positioned to receive the first optical signal component 241 a afterit propagates through the first optical polarization rotator 230. Thethird port 212 is optically coupled with the second polarization rotator231 and positioned to receive the first optical signal component 241 aafter it propagates through the second optical polarization rotator 231.

The first port 210 can receive the first optical signal 240 from atransmitter for coupling into the bi-directional optical link. Thesecond port 211 can couple to a bi-directional optical link. The secondport 211 can transmit the first optical signal 240 to the bi-directionaloptical link to which the second port 211 is coupled. The second port211 can also receive the second optical signal 241 from thebi-directional optical link. The first polarizing beam splitter 220 canbe configured to receive the second optical signal 241. The firstpolarizing beam splitter 220 can also be configured to split the secondoptical signal 241 into the first optical signal component 241 a and thesecond optical signal component 241 b. The second optical signal 241 hasa mixed polarization when it is initially received at the second port211 as denoted by the solid line and the dashed line. When thepolarizing beam splitter 220 splits the second optical signal 241, itsplits the signal into separate optical signal components with differinglinear polarization. Ideally these separate optical signals are fullypolarized after the split, with orthogonal polarizations.

The first polarizing beam splitter 220 can be further configured todirect the first optical signal component 241 a towards the firstreflective surface 221. The first reflective surface 221 can beconfigured to further direct the first optical signal component 241 a topass through a first optical polarization rotator 230 and a secondoptical polarization rotator 231. In some implementations, the firstoptical polarization rotator 230 can be a half-wave plate and the secondoptical polarization rotator 231 can be a Faraday rotator. The thirdport can be configured to transmit the first optical signal component241 a from the integrated optical circulator 200 after it passes throughthe second optical polarization rotator 231. The first optical signalcomponent 241 a, when transmitted from the third port 212, can have afirst polarization state.

The first polarizing beam splitter 220 can also be configured to directthe second optical signal component 241 b through the third opticalpolarization rotator 232 and the fourth optical polarization rotator233. In some implementations, the third optical polarization rotator 232can be a half-wave plate and the fourth optical polarization rotator 233can be a Faraday rotator. The second polarizing beam splitter 222 can becoupled to the fourth optical polarization rotator 233 and configured toreceive the second optical signal component 241 b after the secondoptical signal component 241 b passes through the fourth opticalpolarization rotator. The second polarizing beam splitter 222 can alsobe configured to direct the second optical signal component 241 btowards the second reflective surface 223. The second reflective surface223 can be configured to direct the second optical signal component 241b towards the fifth optical polarization rotator 234. The fourth port213 can be configured to transmit the second optical signal component241 b after it passes through the fifth optical polarization rotator234. The fourth port 213 can be coupled to a second receiver port and beconfigured to transmit the received second optical signal component 241b into the second receiver port. The second optical signal component 241b, when transmitted from the fourth port 213 can have a secondpolarization state substantially aligned with the first polarizationstate. Note that the second optical polarization rotator 231 and thefourth optical polarization rotator 233, which in this example areFaraday rotators, are optional and provide additional performancebenefits if the corresponding receiver ports create high opticalreflection. By ensuring both optical signal components are substantiallyaligned when they are transmitted at the third port 212 and the fourthport 213, the circulator can reduce signal loss.

The first optical signal 240 is p-polarized when it is received by thefirst port 210. This is because most semiconductor lasers arep-polarized. Consequently, the first optical signal 240 is fullypolarized and can propagate through the various polarizing beamsplitters and rotators to be received by the second port.

Another advantage of the present circulator is that it eliminates theneed for an optical isolator. Most transceivers today utilize opticalisolators, which are placed in front of or downstream from thetransmitter in order to redirect reflection signals external to thetransceiver away from the transmitted laser. This is because unwantedincoming external optical reflection signals can affect laser operationand stability. However, with the use of the present circulator, thesilicon photonic chip's transmitter port Tx is isolated from opticalsignals, and thus there is no need for an optical isolator. Eliminationof the isolator reduces optical loss and cost, and improves transceiverdensity.

The specific placements of the various circulator components are notlimited to only the configuration shown in FIG. 2. For instance, FIG. 3shows a schematic of an additional example of an integrated opticalcirculator 300 according to an illustrative implementation. Theintegrated optical circulator 300 can be used to implementbi-directional, polarization diverse optical links using siliconphotonics. The integrated optical circulator 300 is similar to theintegrated optical circulator 200 except that optical polarizationrotators 230 and 232 are one contiguous optical polarization rotator,and optical polarization rotators 231 and 233 are also one contiguousoptical polarization rotator. The integrated optical circulator 300includes a first port 310, a second port 311, a third port 312, and afourth port 313. The integrated optical circulator also includes a firstpolarizing beam splitter 320, a first reflective surface 321, a secondpolarizing beam splitter 322 and a second reflective surface 323. Theintegrated optical circulator also includes a first optical polarizationrotator 330, a second optical polarization rotator 331, and a thirdoptical polarization rotator 332.

The first port 310 can receive a first optical signal 340 from atransmitter for coupling into the bi-directional optical link. Thesecond port 311 can couple to a bi-directional optical link. The secondport 311 can transmit the first optical signal 340 to the bi-directionaloptical link to which the first port 311 is coupled. The second port 311can also receive a second optical signal 341 from the bi-directionaloptical link. The first polarizing beam splitter 320 can be opticallycoupled to the second port 311. The first polarizing beam splitter 320can be configured to receive the second optical signal 341. The firstpolarizing beam splitter 320 can also be configured to split the secondoptical signal 341 into a first optical signal component 341 a and asecond optical signal component 341 b.

The first polarizing beam splitter 320 can be further configured todirect the first optical signal component 341 a towards the firstreflective surface 321, wherein the first reflective surface 321 isconfigured to further direct the first optical signal component 341 a topass through the first optical polarization rotator 330 and the secondoptical polarization rotator 331. In this example, the first opticalpolarization rotator 330 is a half-wave plate and the second opticalpolarization rotator 331 is a Faraday rotator. The third port 312 can beconfigured to transmit the first optical signal component 341 a after itpassed through the second optical polarization rotator 331. The thirdport can also be coupled to a first receiver port and be configured totransmit the received first optical signal component 341 a into thefirst receiver port. The first optical signal component 341 a, whentransmitted from the third port 312, can have a first polarizationstate.

The first polarizing beam splitter 320 can further be configured todirect the second optical signal component 341 b through the firstoptical polarization rotator 330 and the second optical polarizationrotator 331. The second polarizing beam splitter 322 can be configuredto receive the second optical signal component 341 b after the secondoptical signal component 341 b passes through the second opticalpolarization rotator 331. The second polarizing beam splitter 322 canalso be configured to direct the second optical signal component 341 btowards the second reflective surface 323. The second reflective surface323 can be configured to direct the second optical signal component 341b towards the third optical polarization rotator 331. The fourth port313 can be configured to transmit the second optical signal component341 b after it passes through the third optical polarization rotator332. The fourth port 313 can also be coupled to a second receiver portand be configured to transmit the received second optical signalcomponent 341 b into the second receiver port. The second optical signalcomponent 341 b, when transmitted from the fourth port 313 can have asecond polarization state substantially aligned with the firstpolarization state. Note that the second optical polarization rotator331, which in this example is a Faraday rotator, is optional andprovides additional performance benefits if the corresponding Rx1 andRx2 ports create high optical reflection.

Notice that integrated optical circulator 300 is similar to integratedoptical circulator 200 except that optical polarization rotator 230 andoptical polarization rotator 232 are now one continuous opticalpolarization rotator 330. Additionally, optical polarization rotator 231and optical polarization rotator 233 are now one continuous opticalpolarization rotator 331. This design allows for efficient manufacturingof the integrated optical circulator as all the components can now beeasily stacked and divided into multiple integrated optical circulators.

Furthermore, the integrated optical circulator can be used inmulti-channel devices, where multiple transmitters and receivers areintegrated on a single silicon photonic chip. As the integrated opticalcirculator is a free space optical component, vertical displacement ofthe incoming and outgoing optical signals allow for the reuse andreplication of the circulator's components and their function formultiple channels. Thus, a single integrated optical circulator can beused for two or more transmitter/receiver ports of a silicon photonicchip.

For instance, FIG. 4 shows a schematic of an additional example of anintegrated optical circulator 400 according to an illustrativeimplementation. The integrated optical circulator 400 can be used toimplement bi-directional, polarization diverse optical links usingsilicon photonics with multiple transceivers. The integrated opticalcirculator 400 is similar to the integrated optical circulator 200except that integrated optical circulator 400 is configured to receiveand separate additional optical signals. The integrated opticalcirculator 400 includes a first port 410, a second port 411, a thirdport 412, a fourth port 413, a fifth port 410 a, a sixth port 411 a, aseventh port 412 a and an eighth port 413 a. The integrated opticalcirculator also includes a first polarizing beam splitter 420, a firstreflective surface 421, a second polarizing beam splitter 422 and asecond reflective surface 423. The integrated optical circulator alsoincludes a first optical polarization rotator 430, a second opticalpolarization rotator 431, a third optical polarization rotator 432, afourth optical polarization rotator 433 and a fifth optical polarizationrotator 434.

The first port 410 and the fifth port 410 a can receive a first opticalsignal 440 and a third optical signal 442 respectively, wherein bothoptical signals are received from transmitters for coupling into thebi-directional optical link. The second port 411 and the sixth port 411a can couple to bi-directional optical links. The second port 411 andthe sixth port 411 a can transmit the first optical signal 440 and thethird optical signal 442 respectively. The second port 411 and the sixthport 411 a can transmit the received optical signals to thebi-directional optical links to which they are coupled. The second port411 and the sixth port 411 a can also receive a second optical signal441 and a fourth optical signal 443 from the bi-directional opticallinks to which they are coupled. The first polarizing beam splitter 420can be optically coupled to the second port 411 and the sixth port 411a. The first polarizing beam splitter 420 can be configured to receivethe second optical signal 441 and the fourth optical signal 443. Thefirst polarizing beam splitter 420 can also be configured to split thesecond optical signal 441 into a first optical signal component 441 aand a second optical signal component 441 b. The first polarizing beamsplitter 420 can further be configured to split the fourth opticalsignal 443 into a third optical signal component 443 a and a fourthoptical signal component 443 b.

The first polarizing beam splitter 420 can be further configured todirect the first optical signal component 441 a and the third opticalsignal component 443 a towards the first reflective surface 421, whereinthe first reflective surface 421 is configured to further direct thefirst optical signal component 441 a and the third optical signalcomponent 443 a to pass through a first optical polarization rotator 430and a second optical polarization rotator 431. In this example, thefirst optical polarization rotator 431 is a half-wave plate and thesecond optical polarization rotator 431 is a Faraday rotator. The thirdport 412 and the seventh port 412 a can be configured to transmit thefirst optical signal component 441 a and the third optical signalcomponent 443 a respectively after they pass through the second opticalpolarization rotator 431. The third port 412 and the seventh port 412 acan also be coupled to a first receiver port and a second receiver portrespectively. The third port 412 can be configured to transmit the firstoptical signal component 441 a into the first receiver port. The seventhport 412 a can be configured to transmit the third optical signal 443 ainto the second receiver port. The first optical signal component 441 aand the third optical signal component 443 a, when transmitted from thethird port 412 and the seventh port 412 a respectively, can have a firstpolarization state.

The first polarizing beam splitter 420 can also further be configured todirect the second optical signal component 441 b and the fourth opticalsignal component 443 b through the third optical polarization rotator432 and the fourth optical polarization rotator 433. In this example,the third optical polarization rotator 432 is a half-wave plate and thefourth optical polarization rotator 433 is a Faraday rotator. The secondpolarizing beam splitter 422 can be configured to receive the secondoptical signal component 241 b and the fourth optical signal component443 b after they pass through the fourth optical polarization rotator433. The second polarizing beam splitter 422 can be configured to directthe second optical signal component 441 b and the fourth optical signalcomponent 443 b towards the second reflective surface 423. The secondreflective surface 423 can be configured to direct the second opticalsignal component 441 b and the fourth optical signal component 443 btowards the fifth optical polarization rotator 434. The fourth port 413can be configured to transmit the second optical signal component 441 bafter it passes through the fifth optical polarization rotator 434. Thefourth port 413 can also be coupled to a third receiver port and beconfigured to transmit the second optical signal component 441 b intothe third receiver port. The eighth port 413 a can configured totransmit the fourth optical signal component 443 b after it passesthrough the fifth optical polarization rotator 434. The eighth port 413a can also be coupled to a fourth receiver port and be configured totransmit the received fourth optical signal component 443 b into thefourth receiver port. The second optical signal component 441 b and thefourth optical signal component 443 b, when transmitted from the fourthport 413 and the eighth port 413 a respectively, can have a secondpolarization state substantially aligned with the first polarizationstate.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular implementations of particularinventions. Certain features that are described in this specification inthe context of separate implementations can also be implemented incombination in a single implementation. Conversely, various featuresthat are described in the context of a single implementation can also beimplemented in multiple implementations separately or in any suitablesub-combination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asub-combination or variation of a sub-combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemscan generally be integrated together in a single software product orpackaged into multiple software products.

References to “or” may be construed as inclusive so that any termsdescribed using “or” may indicate any of a single, more than one, andall of the described terms. The labels “first,” “second,” “third,” andso forth are not necessarily meant to indicate an ordering and aregenerally used merely to distinguish between like or similar items orelements.

Various modifications to the implementations described in thisdisclosure may be readily apparent to those skilled in the art, and thegeneric principles defined herein may be applied to otherimplementations without departing from the spirit or scope of thisdisclosure. Thus, the claims are not intended to be limited to theimplementations shown herein, but are to be accorded the widest scopeconsistent with this disclosure, the principles and the novel featuresdisclosed herein.

What is claimed is:
 1. An integrated optical circulator enablingpolarization diversity, the integrated optical circulator comprising: afirst port configured to receive a first optical signal; a second portconfigured to transmit the first optical signal received at the firstport and also configured to receive a second optical signal; a firstpolarization beam splitter configured to: receive the second opticalsignal; split the second optical signal into a first optical signalcomponent and a second optical signal component; direct the firstoptical signal component towards a first reflective surface wherein thefirst reflective surface is configured to further direct the firstoptical signal component to pass through a first optical polarizationrotator; and direct the second optical signal component through a secondoptical polarization rotator; a second polarization beam splitterconfigured to receive the second optical signal component after thesecond optical signal component passes through the second opticalpolarization rotator, and direct the second optical signal componenttowards a second reflective surface, the second reflective surfacedirecting the second optical signal component towards a third opticalpolarization rotator; a third port configured to transmit the firstoptical signal component of the second optical signal after it passesthrough the first optical polarization rotator, the third porttransmitting the first optical signal component at a first polarizationstate; and a fourth port configured to transmit the second opticalsignal component of the second optical signal after it passes throughthe third optical polarization rotator, the fourth port transmitting thesecond optical signal component at a second polarization statesubstantially aligned with the first polarization state.
 2. Theintegrated optical circulator of claim 1 wherein at least one opticalpolarization rotator is a half-wave plate.
 3. The integrated opticalcirculator of claim 1, wherein the first optical polarization rotatorand the second optical polarization rotator are different portions of acontinuous optical polarization rotator.
 4. The integrated opticalcirculator of claim 1, wherein: a fifth port is configured to receive athird optical signal; a sixth port is configured to transmit the thirdoptical signal received at the fifth port and also configured to receivea fourth optical signal; the first polarization beam splitter is furtherconfigured to: receive the fourth optical signal; split the fourthoptical signal into a third optical signal component and a fourthoptical signal component; direct the third optical signal componenttowards the first reflective surface wherein the first reflectivesurface is configured to further direct the third optical signalcomponent to pass through the first optical polarization rotator; anddirect the fourth optical signal component through the second opticalpolarization rotator; the third polarization beam splitter is configuredto receive the fourth optical signal component after the fourth opticalsignal component passes through the second optical polarization rotator,and direct the fourth optical signal component towards the secondreflective surface, the second reflective surface directing the fourthoptical signal component towards a third optical polarization rotator; aseventh port configured to transmit the third optical signal componentafter it passes through the first optical polarization rotator, theseventh port transmitting the third optical signal component at thefirst polarization state; and an eighth port configured to transmit thefourth optical signal component after it passes through the thirdoptical polarization rotator, the eighth port transmitting the fourthoptical signal component at the second polarization state.
 5. Theintegrated optical circulator of claim 1, wherein at least oneadditional optical polarization rotator is optically coupled to eitherthe first optical polarization rotator, the second optical polarizationrotator, or both.
 6. The integrated optical circulator of claim 5,wherein the at least one additional optical polarization rotator is aFaraday rotator.
 7. The integrated optical circulator of claim 1,wherein the third reflective surface is a polarizing beam splitter. 8.The integrated optical circulator of claim 1, wherein the fourthreflective surface is a polarizing beam splitter
 9. The integratedoptical circulator of claim 1, wherein the first polarization state andsecond polarization state are p-polarizations.
 10. The integratedoptical circulator of claim 1, wherein the first transmitted signal isp-polarized.
 11. The integrated optical circulator of claim 1, whereinthe first polarizing beam splitter splits the second optical signal intoa first optical signal component and a second optical signal componentcarrying orthogonal polarizations.